Science: Neptune runs rings around the astronomers

AT THE end of August, the space probe Voyager 2 passed Neptune. Now,
the scientists have had a chance to analyse in detail some of the 9000 images
and other information. The preliminary results are published this week,
and fill 85 pages of Science.

The most mysterious aspect of Neptune now appears to be its system of
rings. As Voyager approached Neptune, its cameras revealed two narrow rings
around the planet, with radii of 53 200 and 62 900 kilometres from the planet’s
centre. The outer ring contains three regions which are much brighter than
the rest of the ring, and these have been called arcs.

After Voyager passed Neptune, and looked back towards the Sun, its cameras
showed the rings more clearly, because of sunlight scattered forwards by
small particles of dust – rather as dust on a car windscreen looks bright
as you drive into the Sun. These images showed a broad ring closer into
the planet, 1700 kilometres wide, at a radius of 41 900 kilometres from
Neptune’s centre. They also showed a very faint sheet of material extending
from halfway between the two narrow rings downwards, through the inner narrow
ring and the broad ring, possibly to Neptune’s cloud tops.

Voyager’s cameras showed that the outer ring is very narrow, no more
than 15 kilometres wide. The mission controllers had, however, arranged
a more sensitive measurement of the ring’s width, using Voyager’s photopolarimeter,
a device that measures the intensity of ultraviolet radiation. This device
watched a star, Sigma Sagittarii, as the ring moved in front of it. The
ring dimmed the star’s light for about a tenth of a second. The measurements
show that the ring has a dense region 10 kilometres thick, which probably
corresponds to the width ‘seen’ by the cameras. But the photopolarimeter
also showed a fainter region just outside this narrow ring, giving the ring
a total width of 50 kilometres.

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The Voyager scientists point out that this structure is similar to that
found in the F-ring of Saturn and in two of Uranus’s rings. Saturn’s F-ring
achieved notoriety because it has a twisted or braided structure. Although
Neptune’s ring is not as bizarre,it is similar in having the arc regions
thatare more densely populated than the rest ofthe ring.

The photopolarimeter team points out that Saturn’s F-ring and the rings
of Uranus with a narrow core are ‘shepherded’ by small satellites. Voyager
found six new moons around Neptune. The team suggests that 1989N4, which
orbits 1000 kilometres within the ring, may be responsible for shepherding
the matter in Neptune’s outer narrow ring.

But the team that analysed the images from Voyager points out that it
is still difficult to explain the arcs within this ring. The main problem
is that they seem to have existed for several years. In 1984 and twice in
1985, astronomers on the Earth saw stars disappear as Neptune moved near
them.

This means that the arcs have stayed much as we see them for at least
five years. Yet the particles that make them up should be moving at slightly
different speeds, and should have spread all the way around the ring in
much less than five years, smoothing out the arc structure. It looks as
though some force must be keeping them bunched into the arcs.

Because the satellite 1989N4 follows a very circular orbit, it cannot
shepherd the matter that makes up the ring into particular regions of the
ring to form arcs. The innermost satellite has an orbit that is tilted relative
to the rings. This means it could, in principle, concentrate matter in one
part of the ring, but it is too far from the ring to have such an effect.
Theorists had suggested that a satellite 60 degrees further round in the
same orbit could cause matter to accumulate in arcs, but Voyager did not
find any satellites that were in the right place. The team concludes ‘there
are no theories explaining ring arcs that are verified in Voyager imaging
data’.

The Voyager researchers say that the ring systems of Neptune and Uranus
are superficially fairly similar. Uranus has a set of nine narrow rings,
and large regions of dust. In both cases, the matter making up the rings
consists of particles of ice that are very dark, reflecting only a few per
cent of the light falling on them. This suggests that they formed by similar
processes.

But there are important differences, too. Neptune’s ring system contains
a lot less mass in total, only 0.0001 as much as there is in Uranus’s rings.

The ring systems of Uranus and Neptune both lie within the Roche limit,
the distance from the planet where the different gravitational pull on the
near and far side of a satellite would pull it apart, if the matter of the
satellite were not held together by atomic forces. What this means in practice
is that a moon that is disrupted this close to the planet will not recombine,
but will form a ring of debris.

Another difference between Uranus and Neptune lies in the satellites
that orbit within the Roche limit. Neptune has five satellites in this region,
with diameters between 55 and 190 kilometres. Scientists have searched the
Voyager images carefully for other satellites, and have concluded there
are no more moons in this region larger than 12 kilometres in diameter.
Uranus has rather more satellites within its Roche limit – nine – but they
are about half the size of Neptune’s.

The Voyager team calculates that if it were to put together all the
material – rings and satellites – within the Roche zone of Saturn, Uranus
and Neptune, they would form single bodies with diameters respectively of
390, 150 and 260 kilometres.

The bright rings of Saturn probably formed when a large icy body from
outside Saturn’s system disrupted an icy moon, and astronomers think that
similar collisions could account for Uranus’s rings. But there are problems
in applying this mechanism for Neptune.

Voyager’s images show that Neptune’s two narrow rings contain a very
large amount of fine dust: up to three-quarters of its matter may be in
the form of microscopic particles. The main rings of Saturn and Uranus,
in comparison, are less than 1 per cent dust. If Neptune’s rings were caused
by the influx of bodies from outside the system then there must be more
than a hundred times as many objects hitting the environs on Neptune as
there are in the case of the other outer planets.

The team suggests that, instead, the dust comes from collisions of bodies
with the rings. One of Voyager’s images may be showing us just such collisions
in action. A picture of one of the arcs shows streaks that appear to be
objects moving around in the ring. At first, these were called ‘moonlets’;
but as they appear brighter in light scattered forward they must contain
dust particles. The team thinks these may be ‘clumps of debris’.